1. Field of the Invention
The present invention relates to a light condensing device that condenses light having entered through a front surface thereof along the direction running toward a side surface, and a photovoltaic power generation device and a photo-thermal conversion device both achieved by adopting the light condensing device.
2. Description of Related Art
A light condensing device known in the related art that condenses light having entered through a front surface onto a side surface, includes a prism member having an entrance surface through which solar light enters and a bottom surface that intersects the entrance surface at an acute angle. The prism member condenses the solar light having entered through the entrance surface by causing it to be reflected a plurality of times between the bottom surface and the entrance surface.
FIG. 22 illustrates the basic concept of the light condensing device described above. The light condensing device 9 is configured with a prism member 90 having an entrance surface 91 through which solar light enters, a bottom surface 92 that intersects the entrance surface 91 by forming a vertex angle ε and reflects the incident light and an exit surface 95 that intersects the entrance surface 91 substantially at a right angle. The prism member 90 is formed so as to achieve a prismatic or wedge-shaped section. A photovoltaic power generation device is configured by disposing a photoelectric conversion element (solar cell) 5 at the exit surface 95, where the condensed light undergoes photoelectric conversion.
Assuming that the prism member 90 has a length L, measured along the horizontal direction, a height H, measured along the thickness direction thereof and a uniform width measured along the direction running perpendicular to the drawing sheet, the light condensing multiplication factor of this light condensing device 9 is generally defined as; (entrance surface area)/(exit surface area)=L/H. In this description, the light condensing multiplication factor defined based upon the geometric shape of the light condensing device as described above will be referred to as a “shape-defined light condensing multiplication factor”. The expression above indicates that in order to improve the shape-defined light condensing multiplication factor, the vertex angle ε must be set smaller.
However, when a smaller vertex angle ε is formed, the angle of incidence with which the light, having entered through the entrance surface 91, enters the bottom surface 92 also becomes smaller. Under such circumstances, the incident light will be transmitted through the bottom surface 92 unless solar light enters the entrance surface 91 with a large angle of incidence θ within the drawing sheet surface on which FIG. 22 is presented. On the contrary, solar light enters the entrance surface 91 at a larger angle of incidence θ, the light intensity (energy density) of the solar light entering through the entrance surface 91 per unit area is bound to be lowered. As a result, it is difficult to efficiently condense the optical energy in the solar light.
A light condensing device proposed in the related art includes a bottom surface 92 formed as a mirror surface by vapor-depositing a metal film at the bottom surface 92 or bonding a reflecting mirror at the bottom surface 92 and condenses incident light onto an exit surface 95 by alternately causing specular reflection at the bottom surface 92 and total reflection at the entrance surface 91, so as to address this issue (see, for instance, Japanese Laid Open Patent Publication No. H6-275859).